Field
This disclosure relates to split-cycle engines incorporating numerous refinements and design features that may generally enhance engine performance. Particularly, this disclosure may increase split-cycle engine compression ratio. It may also raise working fluid temperature differentiation by providing cooler working fluid during the compression stroke, and hotter working fluid during the expansion stroke. Those improvements may be achieved by reducing dead volume usually residing within the various components of a split-cycle engine and connecting tube which serves as fluid connection passage between the compression cylinder (cold) outlet and the expansion cylinder (hot) inlet. Reduced dead volume may enable utilizing higher compression ratios which, in turn, leads to higher power density output and improved efficiency. Having a higher compressed working fluid enables a more efficient heat transfer in an external combustion engine (EC engine).
Description of Related Art
An EC engine (such as a Stirling engine, for example) uses temperature-difference between its hot cylinder and its cold cylinder to establish a close-cycle of a fixed mass of working fluid, which is heated and expanded, and cooled and compressed, thus converting thermal energy into mechanical energy. The greater the temperature-difference between the hot and cold states of the working fluid, the greater the thermal efficiency. The maximum theoretical efficiency is derived from the Carnot cycle; however the efficiency of a real engine is less than this value due to various losses.
A Stirling engine compared to steam engines and internal combustion engines is noted for its potential high efficiency, its quiet operation, and the ability to use almost any heat source or fuel for its operation. This compatibility with alternative and renewable energy sources has become increasingly significant as the price of fossil fuels rises, and also in light of concerns such as climate change and limited oil resources.
A Stirling engine (with and without a regenerator) has a connecting pipe between the cold and hot cylinders. The volume of this pipe, often regarded as “dead volume,” causes a major efficiency loss. Consider an ideal Stirling engine connected to a dead volume via piping. During the high pressure part of the cycle, hot air from the engine mixes with colder air in the dead volume, which leads to a loss in efficiency. This is also true during the low pressure part of the cycle, as warm air mixes with the cooler air at the part of the engine where compression takes place. The same would apply to any other dead volume, such as dead volume within the displacer chamber. To clarify, mixing colder and warmer air together increases entropy but decreases exergy.
To address these problems, a regenerator (or economizer as Robert Stirling called it), was developed to increase the efficiency of Stirling engines. The design was originally a mass of steel wire located in the annulus that absorbed excess energy as the working fluid passed through it. A regenerator is essentially a pre-cooler, reducing the thermal load on the main cooler, as well as a pre-heater, reducing the energy required by the main heater to heat the working fluid.